You have exactly the right idea. You run two separate risers or runs of tubing (can be pex or copper, I usually use copper but PEX is fine), one for the supply, the other for the return, up and under the tub. Stub them out vertically. Do the work wherever work needs to be done, leaving access to the stubs. Do the radiant loop under the tile, stubbing out the ends of the loop under the tub. Then add a coupling to the supply riser, add an air valve, then another coupling, then connect your radiant loop. The couplings and the air valve under the tub will be accessible which is an added bonus. If you chose to not use the air valve, no worries. And as far as "valve" valves being buried in the mud bed, yeah, that makes no sense doing that. All I bury in mud is PEX. No unions, just PEX. You do want the loop to be as clean as possible in terms of it having as few connectors as possible. Again, not a scare tactic, not anything to fret over or fire him over. It's just an added layer of protection in what I consider to be a properly designed loop. Maybe he has the air vent somewhere else in the system. Question, or comment: Baseboard water typically runs at 180 degrees. RFH water at 110-120 degrees. So he should have a mixing valve or make some other accommodations to step the boiler water down from baseboard temps to RFH temps.

How old is the water heater? A wet floor could indicate the tank has sprung a leak, although that wouldn't be too likely unless the tank is more than 8 to 10 years old (but still possible). A leak could extinguish the pilot light and would very likely interfere with the burner firing. Either situation would result in no hot water. If the tank isn't leaking, it's also possible that the pilot has gone out. Are you sure you have propane in your tank? The pilot could also go out if a thermocouple fails, if there was a strong gust of air, or for several other reasons. Did you open the door on the water heater to check if the pilot was burning? If the pilot isn't burning, and you don't see evidence of a leak, try relighting the pilot. There should be instructions on the water heater. If it relights but won't stay on, it's probably the thermocouple, an inexpensive part, but you'll need someone who knows how to replace it. If the pilot won't relight, chances are there's something wrong with the propane supply, call your propane supplier. You're right, there are no elements in a propane water heater. Just a burner and pilot light at the bottom.

Thanks Jake for your reply. The model is Aquastar Bosch 125B LP

You actually want to run them in parallel. Right now, there's nothing to control where the hot water flow comes from at a given fixture, besides the piping losses (length/size of pipe, fittings). If you have a bigger line to/from the bigger heater, and it's closer to the cold water entrance to the house, hot water will want to come through it preferentially. You can get some brands (I know you can for Noritz) where they communicate between 2 heaters in parallel. But I believe that's really intended for where they're sitting next to each other. With them on the opposite sides of the house, you'll probably need to do what we did. We have a tankless on one end of the house for 2 bathrooms. The older gas tank heater is in the garage on the opposite corner of the house. In the crawlspace, there's a closed valve on the hot water line in between the 2 bathrooms and the rest of the house. Basically, this zones the hot water system. If either heater is out, I can open the valve and feed the whole house. But the 2 heaters don't fight each other under normal circumstances. To get your smaller heater to work, you need a closed valve somewhere. It might be on the extension/connection you mentioned in your OP. But that depends on how much stuff is on the 2 systems. I may be wrong, but I think zl700's check-valve solution would still lead to zero flow through one unit most of the time. That'd be the smaller one, since it sounds like it's running the least now.